Fiber types in the mouse levator auris longus muscle: A convenient preparation to study muscle and nerve plasticity

The histochemical composition of the levator auris longus (LAL) muscle has been investigated in adult NMRI mice. Histochemical reaction for myofibrill ar adenosine triphosphatase (ATPase) after preincubation in alkaline and ac idic media, nicotine amideadenine-dinucleolide dehidrogenase (NADH -dehydro genase), and alpha-glycerophosphate dehydrogenase were performed on cryosec tions of LAL muscle. Expression of myosin heavy chain (MyHC) isoforms was d etected with the immunoperoxidase method applying monoclonal antibodies aga inst MyHC isoforms -1, -2a, -2x/d, and -2b, as well as by sodium dodecylsul fate (SDS) glycerol gel electrophoresis. The muscle was proven to be a pure fast-twitch muscle. The most numerous fibers in LAL muscles contained MyHC -2b and some MyHC-2a. Histochemically, pure IIA fibers with oxidative metab olism and pure IIB fibers with glycolytic metabolism were detected. In cont rast to the majority of mature control muscles, numerous hybrid fibers coex pressing MyHC-2x/d with MyHC-2a or MyHC-2b were present. Both hybrids were oxidative-glycolytic; additionally, some hybrids containing MyHC-2a were ox idative. In one out of six muscles, traces of MyHC-1 were detected both wit h immunoperoxidase staining and with SDS glycerol gel electrophoresis. Rare fibers that exceptionally expressed small amounts of MyHC-1 always coexpre ssed MyHC-2a, which is an additional proof that pure type I fibers do not e xist in LAL. Due to these histochemical characteristics and to its previous ly described morphological features, the use of the LAL muscle as a model f or various studies, particularly muscle and nerve interactions, is emphasiz ed. J. Neurosci. Res. 59:692-697, 2000. (C) 2000 Wiley-Liss, Inc.